This invention relates to electrically conductive resin blends and methods for preparing them. More particularly, it relates to the preparation of conductive blends which are ductile and have excellent paint adhesion.
Blends of polyphenylene ethers and polyamides are currently in wide use for the fabrication of such articles as exterior body parts for automobiles. Their use in such areas is particularly advantageous by reason of the combination of the excellent properties of polyphenylene ethers, which include temperature stability and impact resistance, with those of polyamides, which include solvent resistance. It is known that polyphenylene ether-polyamide blends containing more than a rather low minimum threshold proportion of polyamide are incompatible unless special compatibilization steps are taken, and therefore such blends are usually prepared with the addition of a suitable compatibilizing compound.
It is also well known that external automobile parts must be painted. In recent years, electrostatic powder coating methods of painting are becoming more widely used by reason of their convenience and environmental advantages, particularly minimization of volatile emissions. For powder coating to be successful, it is necessary for the resinous article to have a relatively high surface electrical conductivity.
U.S. Pat. No. 5,484,838 describes a method of increasing thermal conductivity by incorporating electrically conductive carbon black into a polymer blend. Similarly, Japanese Kokai 2/201,811 describes the incorporation of conductive carbon black into polyphenylene ether-polyamide compositions, and more particularly into the polyamide continuous phase thereof. A further constituent of such polyphenylene ether-polyamide compositions is usually an impact modifier, most often a block copolymer of styrene and a diene such as butadiene or isoprene which block copolymer may be subsequently hydrogenated. As described, the conductive compositions are prepared by first blending the carbon black with the polyamide and subsequently introducing the polyphenylene ether, impact modifier and compatibilizer, optionally in combination with polystyrene.
In another known method for producing conductive blends, the initial step combines the polyphenylene ether, compatibilizer and impact modifiers and the polyamide and carbon black are then individually added, typically at successive downstream addition ports in an extruder. This method has the advantage that formation of the compatibilized polyphenylene ether-polyamide blend precedes addition of the carbon black, improving blend morphology.
It has been discovered, however, that such blends are frequently characterized by low ductility; i.e., they are brittle. Moreover, adhesion of electrostatically deposited paints to such blends is erratic. It appears that there are chemical differences between the paints employed in Europe, for example, and those employed in the United States in that the former but not the latter have uniformly high adhesion to the surface of the resin blend.
It is of interest, therefore, to provide electrically conductive polyphenylene ether-polyamide blends with improved ductility and improved adhesion to a wide variety of electrostatically applied paints.